In modern communications systems and radio ranging systems pseudo-random-noise (PRN) codes are often employed. These are waveforms which oscillate almost randomly between two states, with transitions occurring in synchronism with the transitions of a square wave. In communication systems the information-carrying signal is modulated by a PRN code before transmission to the receiver. The receiver utilizes a replica of the code in a demodulation process to recover the information-carrying signal. This process requires that the replica code be synchronized in time with the received code modulation. In radio-ranging systems a code-modulated waveform is transmitted, and the receiver measures the time delay in propagation from the transmitter to the receiver. The time delay is measured by synchronizing the timing of a replica code with that of the received code modulation and measuring the resulting time shift of the replica code with respect to a reference clock.
Use of the PRN codes in such systems offers several advantages. Thus, it provides a convenient method of multiplexing, i.e., using one communication channel for the transmission of several signals by utilizing a unique code for each signal. Moreover, it provides a degree of security by preventing those without knowledge of the code from receiving the communication or ranging information. Further, it provides a degree of protection from jamming and spoofing by allowing the receiver to distinguish the transmitted code-modulated waveforms from jamming signals that are not modulated by the code. Such codes also provide a degree of protection from multipath interference by allowing the multipath signals to be distinguished as arriving later than the directly received signals.
These and other advantages are well known and are described, for example, in such publications and text books as "Spread Spectrum Communications", AGARD Lecture Services No. 58, North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development, May-June, 1975, National Technical Information Service Report AD-766 914; "Synchronization Systems in Communication and Control", William C. Lindsey, Prentice-Hall Inc., Englewood Cliffs, N.J., 1972; and "Telecommunications Systems Engineering", W. C. Lindsey and M. K. Simon, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1973.
A key process in the operation of the aforementioned systems incorporating PRN modulated waveforms is the time synchronization of the replica code at the receiver with the code modulations of the incoming signal. This is often accomplished by means of a delay-locked-loop using analog techniques, as described in the above texts. The delay-locked-loop allows the replica code to be synchronized with the received code in the presence of strong interference from background noise, jamming signals, or signals being multiplexed with other codes.
Several problems arise in such known and conventional implementations of delay-locked loops. First, such loops use analog switches which are expensive to implement with adequate speed and accuracy. Secondly, the analog summation network and loop-filter network used therein are expensive to implement with adequate long-term stability under typical environmental perturbations such as vibration, shock, changes in power supply parameters, changes in temperature, or changes in humidity. Thirdly, they require the use of a voltage-controlled oscillator which is expensive to implement with adequate frequency stability under the aforementioned environmental perturbations. These problems are particularly severe when long loop response times are desired in severe environments.